In the art of digital radar system design it is known how to design and build digital radar systems which have enhanced capabilities for identifying targets that are moving relative to the position of the radar system. A well known physical effect which these digital radar systems use to distinguish received echo signals from targets moving relative to the radar system as opposed to received echo signals from targets which are not moving relative to the radar system is the determination of doppler frequency shift that is superimposed on the frequency of the radar system's transmitter output pulse. This determination is made at the digital radar receiver station by comparing the output frequency of the radar system's transmitter with the frequency of the received echo signal.
Doppler frequency shifts can have a very small value compared to the frequency of the radar's transmitter. This results from the fact that the doppler frequency shift for a radar signal as reflected from a moving target is proportional to the product of the target's radial velocity (i.e. the target's velocity relative to the location of the radar system) times the frequency of the transmitted radar pulse divided by the speed of light. Since the speed of light is a very large number compared to the radial velocity of most targets of interest, the division of the speed of light into the product of radial velocity times the output frequency of the radar can result in a small value for the doppler frequency shift. Due to this situation enhanced identification of moving targets is required beyond that achieved by frequency shift measurements, and to accomplish this enhancement the frequency shift measurements for received signals are compared with previously determined frequency shifts for received signals in such a manner as to cancel echo signals which have non-varying frequency shifts. These non-varying signals are indicative of stationary targets and their cancellation accordingly increases the probability of positively identifying signals from targets moving relative to the radar's location. Radars which utilize phase shift detection and cancellation of phase shifted signals are identified in the art as Moving Target Indicator (MTI) radars. The theory of MTI radars is explained in the book "Introduction to Radar Systems" by Merril I. Skolnik, published by McGraw-Hill Book Company in 1962.
For certain applications the performance of digital MTI radars can be inadequate. A reference which teaches a method for providing sophistication to enhance moving target identification is U.S. Pat. No. 3,775,768. The invention claimed in that patent is based on a comparison of received radar signals with stored electronic signal values to control performance of the radar receiver. Specifically the U.S. Pat. No. 3,775,768 discloses a clutter cancellation notch filter selection means for MTI radars in which the filter characteristic is varied as a function of both the clutter amplitude and the clutter bandwidth. Stored threshold values are used to provide a comparison between clutter amplitude, clutter bandwidth and the desired rejection notch filter characteristics. When a comparison is present, an enabling signal is fed to the clutter rejection filter to alter feedback characteristics, thereby providing the desired notch filter characteristics.
Prior art attempts to improve the performance of MTI radars, however, have not effectively utilized the parameter which is of ultimate interest and value to the users of such radars, i.e. doppler frequency shift, to enhance identification of moving targets and accordingly reduce the presentation of clutter.